192 °] Allen: Plankton of the San Joaquin Eiver 115 



maximum since the bodies of individual Entomostraca are so much 

 larger than those of the other planktonts. Just why the numerical 

 maximum of Entomostraca should fall between pulses of other groups 

 does not so clearly appear. The most obvious explanation is that 

 the abundant food supply furnished by other organisms near July 12 

 led to abundant reproduction of Entomostraca (mainly Cyclops), 

 which in turn reduced the number of other organisms and led to its 

 own decline. The pulse culmination of other organisms at July 27 

 would then be due to rapid recovery from the inroads of the Ento- 

 mostraca. 



On account of insufficiency of records, consideration of light rela- 

 tions does not give very satisfying aid toward an explanation of these 

 pulses. Both our records and those of Mr. Higby, the Stockton 

 weather observer, were made at a particular time of day and so fail 

 to show the day as a whole so far as clouds, etc., were concerned. 

 It is also true that such records fail to show the influence of the wind 

 except for a small part of the day. As the records stand (table 7), 

 the daylight conditions appear to have been too nearly uniform 

 through the series to have had any marked influence on plankton 

 pulses. 



Reference to the lunar cycle for July, 1913, however, suggests the 

 probability of its having a strong influence in this connection. It 

 may be noted that the beginning of the marked rise in production 

 of chlorophyll bearers came on July 10 at the first quarter of the 

 moon, and that the apex of the pulse for these organisms came on 

 July 17, at full moon. The rapid decline thereafter may be easily 

 explained by the unusual abundance of Entomostraca and other preda- 

 tory animals, while the smaller pulse culminating on July 27 might 

 be due to partial recovery from their attacks. The evidence here that 

 the waxing moon brings rapid increase of chlorophyll bearing organ- 

 isms is as strong as could possibly be imagined, since the record covers 

 only a single lunar cycle. It makes one wish that the daily records 

 might have been carried over several lunar cycles in order to find the 

 variations which might be expected. This particular series certainly 

 confirms in a definite way Professor Kofoid's argument (1908) that 

 pulses of chlorophyll bearers, and consequently of other planktonts, 

 tend to accompany increases of lunar light. 



The important features in the records of this series are concerned 

 most directly with the chlorophyll bearing organisms, as just discussed. 

 But, as a matter of interest, brief mention will now be given of the 



